linux-stable.git/fs/dcache.c, branch v3.5

Revert "vfs: stop d_splice_alias creating directory aliases"

2012-06-08T17:34:03+00:00

This reverts commit 7732a557b1342c6e6966efb5f07effcf99f56167 (and commit
3f50fff4dace23d3cfeb195d5cd4ee813cee68b7, which was a follow-up
cleanup).

We're chasing an elusive bug that Dave Jones can apparently reproduce
using his system call fuzzer tool, and that looks like some kind of
locking ordering problem on the directory i_mutex chain.  Our i_mutex
locking is rather complex, and depends on the topological ordering of
the directories, which is why we have been very wary of splicing
directory entries around.

Of course, we really don't want to ever see aliased unconnected
directories anyway, so none of this should ever happen, but this revert
aims to basically get us back to a known older state.

Bruce points to some of the previous discussion at

       http://marc.info/?i=<20110310105821.GE22723@ZenIV.linux.org.uk>

and in particular a long post from Neil:

       http://marc.info/?i=<20110311150749.2fa2be66@notabene.brown>

It should be noted that it's possible that Dave's problems come from
other changes altohgether, including possibly just the fact that Dave
constantly is teachning his fuzzer new tricks.  So what appears to be a
new bug could in fact be an old one that just gets newly triggered, but
reverting these patches as "still under heavy discussion" is the right
thing regardless.

Requested-by: Al Viro 
Acked-by: J. Bruce Fields 
Signed-off-by: Linus Torvalds

vfs: remove unused __d_splice_alias argument

2012-05-31T01:04:54+00:00

Nobody sets want_disconn any more.

Reported-by: Peng Tao 
Signed-off-by: J. Bruce Fields 
Signed-off-by: Al Viro

vfs: stop d_splice_alias creating directory aliases

2012-05-31T01:04:54+00:00

A directory should never have more than one dentry pointing to it.

But d_splice_alias() will add one if it finds a directory with an
already-existing non-DISCONNECTED dentry.

I can't find an obvious reproducer, but I also can't see what prevents
d_splice_alias() from encountering such a case.

It therefore seems safest to allow d_splice_alias to use any dentry it
finds.

(Prior to the removal of dentry_unhash() from vfs_rmdir(), around v3.0,
this could cause an nfsd deadlock like this:

	- Somebody attempts to remove a non-empty directory.
	- The dentry_unhash() in vfs_rmdir() unhashes the dentry
	  pointing to the non-empty directory.
	- ->rmdir() then fails with -ENOTEMPTY
	- Before the vfs_rmdir() caller reaches dput(), an nfsd process
	  in rename looks up the directory by filehandle; at the end of
	  that lookup, this dentry is found by d_alloc_anon(), and a
	  reference is taken on it, preventing dput() from removing it.
	- A regular lookup of the directory calls d_splice_alias(),
	  finds only an unhashed (not a DISCONNECTED) dentry, and
	  insteads adds a new one, so the directory now has two
	  dentries.
	- The nfsd process in rename, which was previously looking up
	  the source directory of the rename, now looks up the target
	  directory (which is the same), and gets the dentry newly
	  created by the previous lookup.
	- The rename, seeing two different dentries, assumes this is a
	  cross-directory rename and attempts to take the i_mutex on the
	  directory twice.

That reproducer no longer exists, but I don't think there was anything
fundamentally incorrect about the vfs_rmdir() behavior there, so I think
the real fault was here in d_splice_alias().)

Signed-off-by: J. Bruce Fields 
Signed-off-by: Al Viro

brlocks/lglocks: API cleanups

2012-05-30T03:28:41+00:00

lglocks and brlocks are currently generated with some complicated macros
in lglock.h.  But there's no reason to not just use common utility
functions and put all the data into a common data structure.

In preparation, this patch changes the API to look more like normal
function calls with pointers, not magic macros.

The patch is rather large because I move over all users in one go to keep
it bisectable.  This impacts the VFS somewhat in terms of lines changed.
But no actual behaviour change.

[akpm@linux-foundation.org: checkpatch fixes]
Signed-off-by: Andi Kleen 
Cc: Al Viro 
Cc: Rusty Russell 
Signed-off-by: Andrew Morton 
Signed-off-by: Rusty Russell 
Signed-off-by: Al Viro

mm: add a low limit to alloc_large_system_hash

2012-05-24T04:28:21+00:00

UDP stack needs a minimum hash size value for proper operation and also
uses alloc_large_system_hash() for proper NUMA distribution of its hash
tables and automatic sizing depending on available system memory.

On some low memory situations, udp_table_init() must ignore the
alloc_large_system_hash() result and reallocs a bigger memory area.

As we cannot easily free old hash table, we leak it and kmemleak can
issue a warning.

This patch adds a low limit parameter to alloc_large_system_hash() to
solve this problem.

We then specify UDP_HTABLE_SIZE_MIN for UDP/UDPLite hash table
allocation.

Reported-by: Mark Asselstine 
Reported-by: Tim Bird 
Signed-off-by: Eric Dumazet 
Cc: Paul Gortmaker 
Signed-off-by: David S. Miller

Revert "vfs: remove unnecessary d_unhashed() check from __d_lookup_rcu"

2012-05-22T01:48:10+00:00

This reverts commit 8c01a529b861ba97c7d78368e6a5d4d42e946f75.

It turns out the d_unhashed() check isn't unnecessary after all: while
it's true that unhashing will increment the sequence numbers, that does
not necessarily invalidate the RCU lookup, because it might have seen
the dentry pointer (before it got unhashed), but by the time it loaded
the sequence number, it could have seen the *new* sequence number (after
it got unhashed).

End result: we might look up an unhashed dentry that is about to be
freed, with the sequence number never indicating anything bad about it.
So checking that the dentry is still hashed (*after* reading the sequence
number) is indeed the proper fix, and was never unnecessary.

Reported-by: Dave Jones 
Signed-off-by: Linus Torvalds

vfs: be even more careful about dentry RCU name lookups

2012-05-21T23:14:04+00:00

Miklos Szeredi points out that we need to also worry about memory
odering when doing the dentry name comparison asynchronously with RCU.

In particular, doing a rename can do a memcpy() of one dentry name over
another, and we want to make sure that any unlocked reader will always
see the proper terminating NUL character, so that it won't ever run off
the allocation.

Rather than having to be extra careful with the name copy or at lookup
time for each character, this resolves the issue by making sure that all
names that are inlined in the dentry always have a NUL character at the
end of the name allocation.  If we do that at dentry allocation time, we
know that no future name copy will ever change that final NUL to
anything else, so there are no memory ordering issues.

So even if a concurrent rename ends up overwriting the NUL character
that terminates the original name, we always know that there is one
final NUL at the end, and there is no worry about the lockless RCU
lookup traversing the name too far.

The out-of-line allocations are never copied over, so we can just make
sure that we write the name (with terminating NULL) and do a write
barrier before we expose the name to anything else by setting it in the
dentry.

Reported-by: Miklos Szeredi 
Cc: Al Viro 
Cc: Nick Piggin 
Signed-off-by: Linus Torvalds

vfs: make it possible to access the dentry hash/len as one 64-bit entry

2012-05-11T02:54:35+00:00

This allows comparing hash and len in one operation on 64-bit
architectures.  Right now only __d_lookup_rcu() takes advantage of this,
since that is the case we care most about.

The use of anonymous struct/unions hides the alternate 64-bit approach
from most users, the exception being a few cases where we initialize a
'struct qstr' with a static initializer.  This makes the problematic
cases use a new QSTR_INIT() helper function for that (but initializing
just the name pointer with a "{ .name = xyzzy }" initializer remains
valid, as does just copying another qstr structure).

Signed-off-by: Linus Torvalds

vfs: move dentry name length comparison from dentry_cmp() into callers

2012-05-11T02:54:35+00:00

All callers do want to check the dentry length, but some of them can
check the length and the hash together, so doing it in dentry_cmp() can
be counter-productive.

Signed-off-by: Linus Torvalds

vfs: do the careful dentry name access for all dentry_cmp cases

2012-05-11T02:54:09+00:00

Commit 12f8ad4b0533 ("vfs: clean up __d_lookup_rcu() and dentry_cmp()
interfaces") did the careful ACCESS_ONCE() of the dentry name only for
the word-at-a-time case, even though the issue is generic.

Admittedly I don't really see gcc ever reloading the value in the middle
of the loop, so the ACCESS_ONCE() protects us from a fairly theoretical
issue. But better safe than sorry.

Also, this consolidates the common parts of the word-at-a-time and
bytewise logic, which includes checking the length.  We'll be changing
that later.

Signed-off-by: Linus Torvalds